Spectrometer apparatus are known and used for measuring and analyzing the spectral, or "color", contents of electromagnetic energy in the frequency range or spectrum of optical wavelengths, defined herein as being from ultra violet, visible, through infra red, that portion of the electromagnetic radiation which produces photo-electric effects, collectively referred to herein as "light." Those kinds of opto-electronic apparatus are used for both imaging application, as by inspecting the spectral reflectance characteristics of a two-dimensional object, and for non-imaging applications, as by analyzing the spectral emissions of thermal radiation by a material subjected to intense heat. The apparatus are also referred to by many different names depending upon the application to which they are put in part and upon the attached accessories, such as spectrograph, spectroscope, spectroreflectometer, spectrophotometer, spectrofluorometer, spectrobolometer and the like, including those others identified hereinafter. For present purposes such apparatus are collectively referred to as "spectrometer apparatus". Such spectrometer apparatus are considered in some detail next, beginning with non-imaging techniques.
Spectrometric measurements of optical radiation are performed basically in two ways; using a dispersing (refracting or a diffracting) element, and by using a filter-based device. Typical resolving powers from 1,000 to 50,000, and 5,000 to 500,000 are obtained using a prism or diffraction grating, respectively. Resolving power of a hundred to several thousands are obtained with a filter. A third method, using an interferometer, with resolving powers from 10,000 to 5,000,000 is reserved for a very high spectral resolving power applications, and is of no interest as background to the present invention.
In the dispersion based approach to spectral measurement, a radiation dispersion device is used to separate the incident polychromatic light into its spectral contents. The spectrally separated light is then projected onto a photodetector to measure the relative intensity in each spectral range. The dispersion device may be a prism or diffraction grating; in either case the spectrally separated light moves in a diverging or spreading beam. After traveling inside an enclosure over a sufficient distance the spectral bands are adequately separated, spatially. The bands of light are then directed, with reflective optics, at the detection device. A single photodetector, such as a solid state detector or a photomultiplier tube, depending on the spectral region and the intensity, may be used to measure the intensity of one narrow spectral band of the incident light. The dispersion system is then rotated slightly by a mechanical device to a new position in order to direct other narrow bands of the light onto the photodetector and the next intensity measurement is taken. In this sequential manner it is possible to scan and measure the complete spectrum of the light bands that the spectrometer is capable of dispersing.
Such an instrument, often referred to as a monochromator, is commercially available, but is large in weight and size. It requires a physically large enclosure for the diverging light to achieve adequate spatial separation of the light spectra. In addition the instrument is delicate, inappropriate for use in harsh environments, and easily falls out of alignment. That instrument, moreover, cannot provide simultaneous measurement of all the spectral contents in the received light.
To overcome the latter limitation, a polychromater is commonly used. In this apparatus the dispersed light may be directed onto a linear array of photodetectors. The array is positioned such that each individual photodetector in the array measures a different band of the dispersed light being received. For similar reasons to those mentioned earlier in regards to the monochromater, such a polychromater is still large in weight and size, and is too delicate for a harsh field environment. Furthermore, none of these instruments has a selectable spectral resolution or band-width. In order to change the instrument's resolution, the dispersion element has to be changed.
For those less skilled in the art, details of the construction of non-imaging spectrometers, spectrographs, spectroradiometers and spectroscopes are presented by Philip N. Slater, in "Remote Sensing; Optics And Optical Systems", published by Addison-Wesley Publishing Company, 1980, Chapter 7; and by H. S. Chen in "Space Remote Sensing Systems", published by Academic Press, 1985, Chapter 5, to which the interested reader may make reference.
A purpose of the present invention, which employs a photodetector array, is to overcome the difficulties associated with the size, weight, and limitation on spectral accuracy in detection due to mechanical motion, the band-width selection, and the delicate structure of the previously described dispersion-based instruments.
The second approach for measuring the spectral contents of the light utilizes optical filters and photodetectors. A single band-pass filter may be placed over a detector to measure a single spectral band of the incident light. In the "poor man's" spectrometer, a multiplicity of band-pass filters, each of which is used in conjunction with one of a multiplicity of detectors, is used to form a multi-channel instrument. Such a device may be used to simultaneously measure several spectral bands of light, such as in the devices presented in U.S. Pat. Nos. 3,973,118 and 3,737,239. Two or three-channel versions of this concept are commonly used in the so-called 2-or 3-color pyrometers for non-contact temperature measurements.
Of the preceding two patents, the method used by LaMontagne in U.S. Pat. No. 3,973,118 measures simultaneous and discrete spectral bands by the use of several detector and spectral band-pass filter combinations, packaged in a single housing. Although in theory, such approach may be expanded to a very large number of detectors, the discrete photodetectors in LaMontagne's device cannot be positioned in the housing with the spatial precision required for imaging applications or for very accurate spectroscopic measurements, those requiring resolution on the order of but a few Angstroms in wavelength. Further, since no known set of discrete optical filters of the type presented in LaMontagne are available, in which each narrow band-pass filter is slightly spectrally shifted as compared to the adjacent filter, the technique is limited for use in low resolution spectral measurements and to non-contiguous bands only.
In imaging applications, the spatial and the spectral resolution require the use of very small individual detectors, which are positioned very close to each other in great accuracy. Therefore, for similar reasons it would not appear possible to make an acceptable imaging array with LaMontagne's method; a large scale integration, LSI, technology must be applied for adequate imaging performance. Yet another severe limitation of this method is that the band-pass of each detector is determined by and cannot be better than the band-pass of the filter. Moreover, on a more practical note, since each photodetector in the LaMontagne device is connected to an associated external connector pin, a spectrometer device with a very large number of pins, for example, in the thousands or millions, as would be required for high resolution, requires an electrical connector that is completely impractical. Further, since all photodetectors are connected to a common lead, the capacitance of the system with many detectors would be too large to be practical.
The present invention also employs an optical filter. However, as becomes apparent, the limitations characteristic of LaMontangne's device are eliminated in the present invention.
Other variations on the described filter-based technique are common. A filter-wheel, on which several filters are mounted, is used in conjunction with a single photodetector or several photodetectors. The wheel is mechanically rotated to position one filter at a time above the photodetector to provide non-simultaneous, sequential, spectral measurements. Examples of prior knowledge of this form of spectrometer apparatus are presented in U.S. Pat. Nos. 4,477,190; 4,291,985; 4,082,464; 3,963,351; and 3,877,812. In yet another variation, the discrete filters in the disk are replaced with a continuous circular variable filter, "CVF", which is placed over a detector. Continuous or circular variable filters are addressed in the literature: "Circular variable Filters", Yen, Optical Spectra Magazine, 1982; "Have You Considered Using Variable Band Pass Filters", Laser Focus World, September 1989; Circular Variable Filter, Illsley et. al., U.S. Pat. No. 3,442,572. The CVF may be rotated to allow the measurement of a continuous, but not simultaneous, spectrum of light. Alternatively the CVF may be placed over several detectors to provide simultaneous spectra in a limited number of bands, as presented, for example, in U.S. Pat. Nos. 4,657,398; 3,929,398; 3,811,781; and 3,794,425. Finally, a limited number of spectral bands may be obtained by a combination of beam splitters and dichroic filters. This technique is also limited, for practical reasons, to a few bands only. The present invention as is shown hereafter has the spectral measurement capability of hundreds of contiguous bands of light.
Imaging type spectrometers are next considered as additional background. An imaging device which uses a charge coupled device, CCD array, is described by Goetz and Landauer in U.S. Pat. No. 4,134,683. In the Goetz and Landauer invention, four such CCD arrays are used, each in conjunction with a corresponding band-pass filter, to give four overlapping spectral images of the same scene in a method similar to that used in some broadcast television cameras. The difficulty with that approach to spectral imaging, stems from the need to precisely align all four detector arrays with the scenery being viewed. Even if the alignment is correctly accomplished, however, only a four color resolution is obtained. An extension of this technique to additional colors appears impractical because of the alignment difficulties of multiple arrays and the impracticality of simultaneously handling the great amount of data output of dozens or more such arrays to obtain a better spectral resolution. An aspect to the present invention includes the adaption of CCD devices within a versatile high resolution imaging spectrometer, one that is not limited to four color resolution.
Inventions presented in U.S. Pat. No. 4,764,670 and U.S. Pat. No. 4,081,277 each teach how to make solid state imaging devices by depositing a filter array over a multi-sensor device. Both techniques discuss the use of the three primary color filters, red, green and blue. Although presently marketed color video technology utilizes such an approach, it is also limited in the spectral resolution, primarily due to the broadband filters used. It is deficient in the spatial resolution, since it takes at least three photosensitive elements to cover a single point, each at one of the primary colors. Details of imaging systems are given by P. N. Slater and H. S. Chen in the reference cited previously. Embodiments of the present invention provide an essentially "infinite" spectral resolution at no loss of spatial resolution.
Space borne applications for spectrometer apparatus in both imaging and non-imaging application have been extensive. For example, a 12-channel prism, and a 9-channel grating spectrometers were constructed for space-borne sensing of terrestrial resources. A 13-band multispectral scanner, flown on the Skylab, measured spectral bands from 20 to 100 nm wide in the range between 410 nm and 2350 nm.
The LANDSAT-D satellite used two scanning-type instruments, the Thematic Mapper, .TM., and the Multi-Spectral Scanner, MSS. The latter sensor used four channels: a 500 to 600 nm, green; 600 to 700 nm, yellow; 700 to 800 nm, red and near infrared; and 800 to 1100 nm, near IR. The Coastal Zone Color Scanner, CZCS, flown on the Nimbus satellite used a grating spectrometer and five visible-near IR channels with a spectral band of about 20 nm in the visible, centered at 443 nm, blue; 520 nm, green; 550 nm, yellow; and 670 nm, red; a 100 nm band in the near IR centered at 750 nm, and a 2 micrometer band in the far IR centered at 11.5 micrometer. A dichroic beam splitter was used to separate the far IR radiation band from the visible radiation band. In a scanning system, a moving mirror is used to scan different parts of the scene across the array of detectors in order to get multispectral images, each detector operates at a different wave-band.
A 14-channel radiometer, using detector/filter combinations was used on the Earth Radiation Budget, ERB, sensor flown on the Nimbus 7 satellite. The Solar Backscatter Ultraviolet, SBUV, used a moving grating spectrometer to monitor 12 selected narrow wavelength bands, and a filter photometer to measure a fixed band. The Total Ozone Mapping Spectrometer, TOMS, measured six discrete wavelength with 1 nm band. Both instruments have also flown on the Nimbus satellite.
The French SPOT satellite employs two High Resolution Visible, HRV, imaging sensors. The multispectral sensor uses CCD arrays with filter based spectral bands centered at 550 nm, green; 650 nm, red; and 840 nm, near IR, all with about 80 nm band-pass. The panchromatic CCD has a band-pass from 500 to 900 nm. NASA's multispectral Linear Array, MLA, uses four fixed band CCD channels with band-pass from 460 to 470 nm, 560 to 580 nm, 660 to 680 nm, 870 to 890 nm, and two near IR CCDs with fixed bands from 1230 to 1250 nm, and 1540 to 1560 nm.
The Airborne Visible Infrared Imaging Spectrometer, AVIRIS, placed in service by NASA in 1987, is one of the most advanced imaging spectrometers uses 244 bands with about 9.6 nm bandwidth. Finally, the new generation imaging spectrometers scheduled to be constructed and flown by NASA on-board the Space Station in the late 1990s are the High Resolution Imaging Spectrometer, HIRIS, and the Moderate-Resolution Imaging Spectrometer, MODIS. Both instruments use area arrays to obtain spectrally resolved images of a one-dimensional scene. The spectral resolution, though, is achieved with a diffraction grating with a 10 nm band-width. Further particulars of various space-borne instruments are described in a Report to the Congress titled Space-Based Remote Sensing of the Earth, prepared by NOAA and NASA in 1987; in the Nimbus 7 Users' Guide, published by NASA August 1978; in Remote Sensing of the Environment, by J. Lintz and D. S. Simonett, published by Addison-Wesley, 1976; in the Earth Observing System--Instrument Panel Report, volumes IIb, MODIS, and IIc, HIRIS, published by NASA in 1987, and in the two references cited earlier.
The foregoing imaging devices are limited to a few spectral bands, often provide non-simultaneous spectra, and, depending on the application, require a scanning system, which is less reliable and less accurate than a system with non-moving parts. The spectral resolution of these devices is low and is fixed by the hardware design and cannot be changed in operation, thus limiting versatility. As it is shown herein the present invention is believed to alleviate the aforementioned deficiencies, through elimination of all moving parts, and increase the operational capabilities of the measuring methods discussed above through software control of the spectral resolution.
An object of the present invention, therefore, is to provide an improved spectrometer apparatus for use in any of the applications described in the foregoing section that is small, lightweight, rugged and permanently aligned, inexpensive, solid state device of versatile application, suitable for manufacture at least in part using LSI technology so that the size and weight can be at least one thousand times smaller than such conventional spectrometer apparatus.
An additional object is to provide spectrometer apparatus having enhanced spectral and spatial resolution than previously existing filter types without use of dispersion devices and which avoids the need for alignment adjustments in use.
A further object of the invention is to provide a simplified method for measuring simultaneously the contiguous spectra of a polychromatic optical radiation by use of a solid-state array and filter combination without the use of moving parts with the arrays being line type or area type in alternative embodiments. Such spectral measurements may be provided in a narrow-band pass mode, a wide-band pass, a long-wave pass, or a short-wave pass modes and for various portions of the optical spectrum; the ultra-violet, the visible, or the infra-red.
A still further object of the invention is to provide spectrometer apparatus that has a selectable spectral resolution from very fine, several Angstroms, to coarse, several nanometers, by means of an associated signal processing technique.
And, it is an additional object of the invention to provide like benefits in spectrometer apparatus, regardless of whether used for non-imaging applications or in imaging applications.